This invention relates to field-effect transistors and more particularly to field-effect transistors that are grown by molecular beam epitaxial techniques.
One form of field-effect transistor that has recently been proposed in the art is a field-effect transistor where the current channel consists of a delta-doped monoplane between two undoped layers of GaAs. This delta-doped field-effect transistor was described in an article entitled "The .delta.-Doped Field-Effect Transistor," by E. F. Schubert and K. Ploog, Japanese Journal of Applied Physics, Vol. 24, No. 8, August, 1985, pp. L608-L610. In the Schubert et al. article, a 0.35 .mu.m thick undoped GaAs layer is first grown on a semi-nsulating gallium arsenide (GaAs) substrate using molecular beam epitaxy (MBE). A Dirac-delta function-like doping profile is then implemented by interrupting the usual crystal-growth mode of GaAs during MBE by closing the Ga shutter, opening a silicon (Si) shutter and leaving the As shutter open. An As-stabilized crystal surface is thus maintained while the shutter of the Si-cell used for n-type doping is kept open, so that an impurity-growth mode results. During this mode the host crystal does not continue to grow. During growth of the normal crystal the (100) surface of GaAs contains about 6.25.times. 10.sup.14 Ga atoms cm.sup.-2. The two-dimensional Si-concentration used for the delta-doping is smaller than the 10.sup.13 atoms cm.sup.-2. Thus only a small selection of the Ga sites of one monolayer is occupied by the silicon atoms.
Following the delta-doped region a 300 .ANG. thick undoped GaAs layer is grown. Finally source and drain electrodes are provided by alloying AuGe/Ni ohmic contacts at 450 degrees C. for three minutes at predetermined points on the surface of the undoped GaAs layer. A Schottky-gate is formed by a Cr/Au film which is positioned between the source and drain electrodes.
The Schottky delta-doped field-effect transistor is a considerable improvement over prior art field-effect transistors but it in turn could be considerably improved if a superior form of ohmic contact was used in place of the alloyed contacts presently used. As is known in the prior art, alloyed ohmic contacts suffer from the fact that donors with different chemical composition are formed thereby modifying the performance of the device. Alloying is also known to be disruptive in that it produces irregularities in the interface between the metal and the undoped semiconductor material. These irregularities in turn can result in unpredictable behavior for the field-effect transistor using these type contacts.